講演者： 高橋 努（リソスフェア構造解析研究チーム）
S-wave attenuation and random inhomogeneities beneath northern Izu-Bonin arc
Seismic waves at high frequencies (>1Hz) at large travel distance (>100km) usually show incoherent and complex wave trains due to accumulation of multiple scattering and attenuation. S-wave random inhomogeneities and inelastic attenuation are important medium properties to describe such wave trains. Recent studies of wave propagation in random media make it possible to estimate spatial distributions of the power spectral density function of random velocity inhomogeneities and S-wave attenuation (Takahashi et al. 2009; Takahashi 2012). Northern Izu-Bonin arc have three anomalies of strong inhomogeneities beneath the volcanic front at 30-70 km depth (Takahashi et al. 2011). These anomalies show a high correlation with the Quaternary volcano distribution. Meanwhile, S-wave attenuation structure shows two anomalies of high attenuation beneath the volcanic front. High attenuation regions are correlated with low-Vs regions (Obana et al. 2010), and broadly coincide with Hot Fingers (Tamura et al. 2009). Strongly inhomogeneous region without any anomalies of velocity and attenuation is located beneath Myojin-sho. Even though we have not elucidated medium properties of this region, power spectrum of random inhomogeneities clearly suggests that origin of medium inhomogeneities is some kind of volcanic process rather than other inhomogeneities such as fractured structure. In this seminar, I will make a brief introduction of wave propagation in random media, and will discuss medium properties of northern Izu-Bonin arc by comparing with other regions.
Volatile behaviors in an immature subduction zone inferred from boninitic melt inclusions at Bonin-Mariana Arc
Recent study suggests that boninites formed at the immature stage of subduction zone, whereas related arc tholeiites erupted 0-7 Myrs after boninite eruptions at Izu-Bonin-Mariana Arc (Ishizuka et al., 2011, EPSL, v306, p229-240). There must be drastic changes in the subduction system at this period. In order to constrain volatile behaviors of an immature subduction zone, I have analyzed volatile contents and sulfur isotopic ratios (34S/32S) of melt inclusions in Cr-spinel from fore-arc volcanic rocks in Bonin Islands and in Guam using a secondary ion mass spectrometry (SIMS) at Woods Hole Oceanographic Institution. All Cr-spinels are collected at volcanic sand beaches and purified for this study. Boninitic melt inclusions occur in Muko-jima, Chichi-jima and tholeiitic melt inclusions occur in Mukoo-jima and Guam. Cr-spinels in boninite are high in Cr# (mostly 80-90) and low in TiO2 (< 0.1wt%), indicating highly depleted source.
Whereas Cr-spinels in tholeiite vary in Cr# (45-80) and in TiO2 (0.1-1 wt%). Compositions of melt inclusions fully cover compositional range of whole-rock. Some melt inclusions of boninites have MgO higher than 20 wt%, showing that they are very primitive magmas. H2O and CO2 contents of melt inclusions of Muko-jima boninite are high (up to 4 wt%) and low (< 50 ppm), respectively whereas those of Mukoo-jima tholeiite are lower (H2O mostly ~1 wt%) and higher (CO2 up to ~1000ppm). Except H2O, volatiles of boninitic melt inclusions (Cl <500ppm; S ~100ppm) are considerably lower than those in tholeiites (Cl up to 3000ppm; S up to 3000ppm). High S content of tholeiitic melt inclusions may indicate high oxygen fugacity of the magmas. Sulfur isotope data of melt inclusions from boninites show the lightest value that reported from igneous rocks (delta34S = -10 to 0 permil), whereas those of tholeiites (delta34S = +2 to +5 permil) are comparable to reported arc tholeiite data. S source of tholeiite should be mixture of seawater-derived hydrothermal sulfites and mantle sulfide. Whereas S source of boninite can be seawater-derived pyrite, which inorganically precipitated in mantle, because of reduced condition caused by water-mantle reaction during serpentinization. As source of boninite is hydrated hertzbergite, sulfur in the source before the hydration may be negligible. Therefore, all sulfur of boninite may be secondary origin. Assuming open system isotope fractionation, delta34S difference between seawater sulfate (20 permil) and pyrite (-10 to 0 permil) can be explained by pyrite precipitation at ~200degC, which is consistent temperature of serpentinization at subduction zone.
Boninite may be formed by melting of this serpentinite of wedge mantle at an immature stage. Further contaminations by fluid led higher oxygen fugacity at mantle wedge, forming arc tholeiites.
A change of shear wave anisotropy within the marine sedimentary layer associated with the 2011 Tohoku-Oki earthquake
Detection of temporal variations of subsurface seismic structure potentially provides information on time-dependent processes occurring within the Earth. Using auto/cross-correlation functions derived from seismic ambient noise, several studies have detected temporal variations of seismic structure and their relaxations, which are associated with large earthquakes and volcanic eruptions.
In this study, we used ambient seismic noise recorded on 1-year continuous records of broadband ocean bottom seismometers (BBOBSs) that are deployed on the outer rise of the Japan Trench. The observation period contains the time of the 2011 great Tohoku-Oki earthquake (Mw9.0), and the distance between the observation location and the source area of the earthquake is 200-400 km. This condition potentially allows us to estimate temporal variations of seismic structure underneath the seafloor.
As a result for the structure below the BBOBS sites, auto-correlating ambient seismic noise, we found persistent reflections of S waves from the bottom of a ~350-m thick marine sedimentary layer. The two-way travel times of reflected S waves, which vary as a function of the polarization direction, indicate a velocity anisotropy of ~1.7% in the sedimentary layer. The fast direction is estimated to be trench-parallel, possibly due to cracks or normal faults formed by bending of the plate in the outer rise.
For temporal variations, the travel time also shows a coseismic velocity reduction of ~2%, with slightly reduced anisotropy, within the layer. The change gradually recovered to pre-earthquake conditions through 4 months after the earthquake, although recovery was not complete during the period of the observation. Such coseismic changes can be explained either by increases of crack density and crack sphericity within the suddenly stressed sedimentary layer or by channeling and networking of water flow in the strongly shaken sedimentary layer.
場所： 横浜研究所 三好記念講堂
Resistivity image deduced by the marine MT survey at 40 degrees North offshore the Sanriku area
Mega-thrust earthquakes with magnitude (M) 8-9 have repeatedly occurred along subduction zones, and have given disasters with large seismic and tsunami waves to populated coastal cities. One of the important factors for nucleation of such great earthquakes is fluid. The fluid has a role of fault weakening at earthquake.
Magnetotelluric (MT) survey are often used for imaginging a deep fluid distribution under the ground because enhanced electrical resistivity at subsolidus temperatures is principally controlled by the presence of water. The MT surveys revealed low resistivity in damaged zones along active faults on land, and some of those studies found partly resistive and possibly less-fluid condition around the asperity of active fault. In case of subduction zone, it is suggested that a large amount of fluid is released from the plate at depth having high pressure.
In this study, we analyze the marine MT survey data with more dense stations were conducted in 2000, at 40 degrees North offshore the Sanriku area, NW Japan. The purpose of MT experiment is for imaging the resistivity around the asperities of three earthquakes; the 1968 off Tokachi earthquake, 1989 earthquake and the 1994 off Sanriku earthquake.
The most prominent feature obtained by our inversion result is the conductive zone below the Japanese island arc. The shallower boundary of conductor C1 roughly corresponds to the seismic reflector, interpreted as the upper boundary of lower crust of the island arc. Therefore, the C1 can be interpreted as the conductive lower crust. The C1 continues to deeper part, so that the lower half of it also corresponds to the subducting oceanic crust. On the contrary, the overlying zone upon this conductor and eastern shallower part of subducting plate boundary has more resistive zone. These results show that asperities of the 1968 and 1994 earthquakes are located on the transient zone from the conductive to resistive zone.
● IFREE ALL セミナーのお知らせ
Geophysical Network in the Western Pacific region: How we operate? and some topics.
Geophysical observation network has important role as one of infrastructures to progress geodynamics research. IFREE and cooperated institutes deployed and operate the observation network in the western Pacific region which has active subducting process. Some stations of our network locate at Pacific islands in sparse station area. The network operates broadband seismic sensors,geomagnetic sensors and recently high resolution barometers are being installed. Each station is integrated to multi parametric measurement one.
It is general case that modern instruments are easy operation and free of maintenance. In IFREE geophysical network performs special maintenance and calibration for instruments by their unique mechanical design to keep their specification and quality. In this topic, I introduce a taste of real situation for our network operation about broadband seismic equipment which speaker relates mainly.
In this presentation, two short topics about seismic data recorded in our network are also introduced. The data are featured in our instrument's specification and in its geographical situation. Very long period seismic signal is sensitive to ground tilt. At island station, ground tilt is caused by ocean tide. In some case, seismic sensor detects tsunami arrival. Tsunami height and tilt magnitude are fine linear relation and its coefficient may depend on thickness of sub-surface layer.
Second topic is seismicity of Palau area which is one of network stations. According to ISC catalogue, this region is low seismicity although it has trench topography. We practiced campaign small array measurement in Palau Island for about six months. We succeeded to pick up many seismic events in only six month. It is much higher than predicted activity by earthquake catalogue. Hypocenters locate on the trench side and somewhat deep layer. The result implies active subduction process in this area.
To make the most of IFREE geophysical network, the co-research with other parameters, collaboration with regional network and ocean bottom measurement are required. The highly controlled with quality stations will be reference ones in cooperated analysis.